1. Field of the Invention
This invention relates to an optical device which eliminates the difficulties resulting from focusing, and is suitable, for example, as the optical system of an ophthalmologic instrument.
2. Description of the Prior Art
Most optical devices are provided with a portion for achieving the focusing, and as the focusing method, for example, in an eye-fundus camera, there is known a method whereby a camera body containing a film therein is moved relative to the device body, and a method whereby the focusing is realized by movement of the forward portion of a picture-taking lens lying rearwardly of a mirror for introducing an illuminating light or by movement of a lens added to the object side of the picture-taking lens. However, in the case of a device which is provided with a zoom lens or an attachment lens and is capable of magnification changing operation, the method whereby the film is moved is disadvantageous because the focus must be re-adjusted each time the magnification is changed. Accordingly, it is convenient that the focusing portion is disposed on the object side with respect to the lens having a magnification changing function, and in Applicant's prior U.S. application Ser. No. 30,959 now abandoned (see FIG. 1 of the accompanying drawings), a negative focusing lens is provided rearwardly of the photographing stop. In FIG. 1, reference numerals 1 and 2 show an eye to be examined in a model fashion, and 1 designates the eye fundus and 2 denotes the pupil. Reference numeral 3 designates the objective lens of an eye-fundus camera, reference numeral 4 denotes the primary eye-fundus image by the objective lens 3, and reference numeral 5 designates an apertured mirror having an opening in the center thereof. The central opening has the function as a stop, but a separate stop may also be disposed behind this reflecting mirror, and the stop and the pupil 2 or the cornea are conjugate with each other. Designated by 6 is a focusing lens group having a negative power and movable back and forth during focusing. Reference numeral 7 denotes a stationary lens group not for imaging but for aberration correction, reference numerals 8 and 9 designate lens groups movable at the same time and independently of each other during zooming, and reference numeral 10 denotes a positive relay lens group. Reference numeral 11 designates a jump-up mirror which is obliquely disposed during observation and retracted out of the optical path during photography. Reference numeral 12 designates a photographing film, and reference numeral 13 denotes a final eye-fundus image which has been re-imaged by an imaging lens comprising six to ten lenses. Reference numeral 14 designates an optical path changing-over mirror and reference numeral 15 denotes an eyepiece, and these two constitute a finder optical system. A light ray l depicted in a solid line is a principal ray. Relay lens groups 16, 17, a light-intercepting plate 18 having a ring-shaped slit therein, a mirror 19, a condenser lens 20, a photographing light source 21, a condenser lens group 22 and an observation light source 23 together constitute an illuminating system for illuminating the eye to be examined through the apertured mirror 5.
However, in the optical system of the aforementioned prior application, if an attempt is made to cause the light beam to pass effectively in whatever position the focusing lens is, off-axis beams passing through the lenses subsequent to the focusing lens become spaced apart from the optical axis and increase the lens diameters and thus, the thickness of each lens has increased and this has led to an optical system having a great full length. These difficulties have occurred not only in a case where the picture-taking lens is of variable magnification but also in a case where the picture-taking lens is of fixed magnification.